The fourth season opens with Finch, Reese, Shaw and Root adapting to the new identities created to protect them from Samaritan. Meanwhile, the Machine continues to send out numbers, potentially putting the entire group in danger of detection.

The Fringe crew is flummoxed by the strange circumstances surrounding a bomb blast at a Philadelphia train station, but Walter's discovery in the lab and the prospect of more explosions send Olivia and Peter to Iraq for answers.